The present invention generally relates to noise reduction circuits for video signals, and more particularly to a noise reduction circuit which comprises a filter circuit within a feedback loop of a feedback type comb filter and effectively reduces noise within an input video signal so as to improve the picture quality, by varying the feedback ratio responsive to the frequency.
Conventionally, a noise reduction circuit is provided in a luminance signal reproducing system of a helical scan type magnetic recording and/or reproducing apparatus (video tape recorder or VTR), for example, so as to reduce noise within a reproduced luminance signal after a frequency demodulation. For example, in a first conventional noise reduction circuit, the reproduced luminance signal which is reproduced from a magnetic tape and is demodulated in a frequency demodulator, is applied to an input terminal and is passed through a highpass filter so as to obtain only a frequency component of over 1 MHz, for example. An output signal of the highpass filter is passed through a limiter and a coefficient multiplier, and is supplied to a subtracting circuit. The subtracting circuit subtracts the output signal of the coefficient multiplier from the video signal (reproduced luminance signal, for example) applied to the input terminal. The noise which is visually conspicuous to the human eye, is generally concentrated in a low-level part of the high-frequency component. Hence, a video signal in which the visually conspicuous noise is eliminated, is produced from the subtracting circuit and is obtained through an output terminal.
On the other hand, in a second conventional noise reduction circuit, the video signal (reproduced luminance signal, for example) which is applied to an input terminal, is supplied to a 1H delay circuit wherein the video signal is delayed by a delay time of 1H, where H represents one horizontal scanning period. An output delayed video signal is supplied to a first subtracting circuit. The first subtracting circuit subtracts the output delayed video signal of the 1H delay circuit from the video signal applied to the input terminal. In the video signal, information contents which are separated by an interval of 1H are extremely similar to each other, and the so-called vertical correlation (line correlation) exists, as is well known. However, the vertical correlation does not exist for the noise. As a result, a signal made up of the noise and a video signal component having no vertical correlation, is obtained from the first subtracting circuit. The output signal of the first subtracting circuit is subjected to an amplitude limitation in a limiter which has a limiting level in the range of a peak-to-peak value of the noise. An output signal of the limiter is supplied to a second subtracting circuit which subtracts the output signal of the limiter from the video signal applied to the input terminal. Consequently, a video signal in which the noise is greatly reduced, is produced from the second subtracting circuit and is obtained through an output terminal.
Further, there is a third conventional noise reduction circuit comprising a feedback type comb filter. This third conventional noise reduction circuit will be described later in detail in conjunction with a drawing. According to the third conventional noise reduction circuit, a video signal (reproduced luminance signal, for example) is applied to an input terminal, and is supplied to the feedback type comb filter which eliminates the noise and obtains a video signal component having the vertical correlation. An output signal of the feedback type comb filter is supplied to a subtracting circuit which subtracts the output signal of the feedback type comb filter from the video signal applied to the input terminal, so as to obtain a signal made up of the noise included within the video signal and a video signal component having no vertical correlation. The output signal of the subtracting circuit is passed through a lowpass filter which obtains only a low-frequency component of the output signal of the subtracting circuit. The output signal of the feedback type comb filter has a predetermined characteristic after being passed through an equalizer circuit. The output signal of the lowpass filter and an output signal of the equalizer circuit are added in an adding circuit. As a result, a signal in which the noise is eliminated, is produced from the adding circuit and is obtained through an output terminal.
The frequency characteristic of the third conventional noise reduction circuit is flat in a frequency band under a cutoff frequency f.sub.c of the lowpass filter, but has a comb filter characteristic in a frequency band over the cutoff frequency f.sub.c so as to pass frequency components which are natural number multiples of a horizontal scanning frequency f.sub.H. Thus, according to the third conventional noise reduction circuit, it is possible to eliminate the noise in the high-frequency band over the cutoff frequency f.sub.c. Further, it is possible to prevent deterioration in the vertical resolution which is visually conspicuous in the low-frequency band under the cutoff frequency f.sub.c.
However, in a case where the video signal applied to the input terminal has an edge of a large amplitude, a high-frequency component of the edge is obtained from the highpass filter in the first conventional noise reduction circuit described before. Thus, in the first conventional noise reduction circuit, the video signal and the noise in the vicinity of the edge are eliminated by the amplitude limitation performed in the limiter. As a result, there is a problem in that a video signal in which the edge noise still remains in the vicinity of the edge where the amplitude limitation is performed in the limiter, is produced from the subtracting circuit and is obtained through the output terminal.
Especially during a long-time mode of a VTR for home use, in which the recording and reproduction are carried out with respect to a given length of magnetic tape for a time which is longer than the recording and reproducing times during a normal mode by making the track width extremely narrow, the signall-to-noise (S/N) ratio of the reproduced video signal is poor because the track width is narrow and the relative linear speed between the magnetic tape and a head is slow. In addition, the crosstalk from adjacent tracks is large, and the edge noise is visually conspicuous in the reproduced picture. For this reason, the S/N ratio cannot be improved sufficiently according to the first conventional noise reduction circuit.
Further, in the VTR for home use, the noise is also distributed in the low-frequency band under 1 MHz. Since the first conventional noise reduction circuit is only effective with respect to the noise over the cutoff frequency of the highpass filter, it is also impossible to obtain the noise reducing effect with respect to the noise in the low-frequency band at parts other than the edge of the video signal.
In a case where the video signal applied to the input terminal has the vertical correlation, the second conventional noise reduction circuit is superior compared to the first conventional noise reduction circuit in that the second conventional noise reduction circuit can eliminate the edge noise and improve the S/N ratio. However, although the S/N ratio can be improved theoretically by 3 dB, the S/N ratio can only be improved by approximately 1.5 dB to 2.0 dB in actual practice. Moreover, the second conventional noise reduction circuit has a comb filter characteristic which passes frequencies which are natural number multiples of the horizontal scanning frequency f.sub.H to the same extent throughout the entire frequency band. As a result, the vertical resolution becomes deteriorated, and there is a problem in that the deterioration in the vertical resolution is visually conspicuous especially in the low-frequency band.
On the other hand, the third conventional noise reduction circuit is advantageous in that it is possible to reduce the edge noise described before. However, there is a problem in that the low-frequency noise (in the range of 1 MHz) which are visually conspicuous especially in the reproduced picture obtained in the VTR, cannot be reduced in the low-frequency band under the cutoff frequency f.sub.c of the lowpass filter. In this case, it is possible to reduce the low-frequency noise by lowering the cutoff frequency f.sub.c of the lowpass filter to a frequency in the range of 1 MHz, however, a coefficient of a coefficient multiplier within the feedback type comb filter must be set to a large value in order to obtain a desired S/N ratio improvement factor which is greater than the S/N ratio improvement factor obtainable in the second conventional noise reduction circuit. For this reason, the comb filter characteristic becomes sharp, and the vertical resolution is greatly deteriorated in the frequency band over the cutoff frequency f.sub.c of the lowpass filter. The deterioration in the vertical resolution is visually conspicuous in a frequency range of 1 MHz to 2 MHz. Hence, the cutoff frequency f.sub.c of the lowpass filter must inevitably be set to a frequency in the range of 2 MHz to 3 MHz, and it is virtually impossible to improve the S/N ratio in the low-frequency band by the desired improvement factor so as to reduce the low-frequency noise which are visually conspicuous especially in the reproduced picture obtained in the VTR.